The Secret Science of the Hubble Space Telescope's Amazing Images

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With the Hubble Space Telescope's successor, the James Webb Space
Telescope, on schedule to reach outer space in 2018, taking
Hubble's place as NASA's premier eye in the sky, it seems
appropriate to look back on what may become Hubble's most
enduring legacy: its stunning images. Besides the huge amount of
data Hubble has collected since its launch in 1990, the telescope
will likely be remembered most for its gorgeous color shots of
nebulas, galaxies and the early universe, iconic images that
seemed tailor made for magazine covers and bedroom walls.

But throughout the storied history of the Hubble
Space Telescope, the beauty of those color images has
sometimes overshadowed one important question: Where does that
color come from? After all, some of Hubble's amazing photos — and
images from other space telescopes, for that matter — depict
astronomical objects in ultraviolet or infrared light. But the
human eye can't perceive those colors. When people look at a
Hubble image showing these hues, what exactly are they seeing?

One person with answers is Ray Villard, the news chief at
Maryland's Space Telescope Science Institute (STScI), which
operates Hubble. According to Villard, the public often has the
wrong idea about Hubble images. "People assume you're painting by
numbers, but you're not," Villard said. [ Amazing Hubble Space
Telescope Photos: Latest Views ]

The raw
Hubble images, as beamed down from the telescope itself, are
black and white. But each image is captured using three different
filters: red, green and blue. The Hubble imaging team combines
those three images into one, in a Technicolor process pioneered
in the 1930s. (The same process occurs in digital SLRs, except
that in your camera, it's automatic.)

Why are the original images in black and white? Because if
Hubble's eye saw in color, the light detector would have to have
red, green and blue elements crammed into the same area, taking
away crucial resolving capability. Without those different
elements, Hubble can capture images with much more detail.

The tricky part is when Hubble uses infrared or ultraviolet
filters. These wavelengths of light, respectively above and below
the visible spectrum,are full of what Villard calls "invisible
colors." Human eyes simply don't see them. Therefore, if
astronomers want to make these images reflect the light's full
spectrum—including ultraviolet and infrared wavelengths – visible
colors have to be added in.

Those added colors aren't random, though. "A common
misconception," noted Villard, "is that when people hear that
color is added, they think that the scientists are like kids with
crayons." On the contrary, astronomers are very careful to stay
as true to nature as they can. Thus, in full-spectrum images, the
details that correspond to infrared light will have the reddest
color and the details corresponding to ultraviolet will have the
bluest. Or, as Zolt Levay, the imaging-resource lead at STScI,
put it, "What is redder in a Hubble image really is
redder."

In short, the
coloring process for Hubble images is not done
willy-nilly."The colors of the images have meaning," said Levay.
"They depend on the data." When people at STScI work with a
full-spectrum Hubble image, they are in essence translating one
kind of light into another so human beings can perceive it.

All of this manipulation begs one enormous question: Why add
color at all? If the coloring process necessarily involves human
manipulation, wouldn't it be better, and safer, to stay with the
original black-and-white versions?

Villard claims the contrary. Color images are "full of
information," he said. "In fact, color is an analytical tool. It
helps the understanding."

The Hubble team uses color in three ways.

First, for objects that would otherwise be too faint for the
human eye to see,the team adds color to make the objects visible.
Second, the team uses color to depict details that the human eye
can't see, like astronomical features only visible in
infrared or ultraviolet light. Third, color can highlight
delicate features that would be otherwise lost.

For example, Hubble took one
1995 image of the Cat's Eye Nebula using three filters: one
that captured light produced by oxygen atoms, one for light
produced by hydrogen atoms and one for light coming from nitrogen
ions. All three colors, though, fall in the red part of the
visible spectrum. To make all of the parts of the nebula as
visible as possible, and to avoid just producing a red mess, the
imaging team made some adjustments. [ Photos:
Hubble's Ring Nebula Portraits ]

They assigned red to the hydrogen light, green to the nitrogen
light and blue to the oxygen light. Human beings then assigned
the colors, but not without a reason behind every decision. As
Levay noted, the coloring process is "a dance between the
subjective (the color that's applied) and the objective (the
data)."

A centuries-old debate among philosophers bears directly on this
issue of astronomical images, color and reality. The question is
whether colors exist in objects and human beings merely see what
is out there in the world, or whether colors are, in a sense,
properties of the mind that arise when human beings perceive
something, and are experienced differently from one individual to
another. Leave it to the Hubble Space Telescope to connect an
ancient intellectual tradition with cutting-edge technology and
the wonders of nebulas, galaxies and burning suns.